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/*
 * Copyright (c) 2000, 2020, Oracle and/or its affiliates.
 *
 * Licensed under the Universal Permissive License v 1.0 as shown at
 * http://oss.oracle.com/licenses/upl.
 */

package com.tangosol.io;


import com.tangosol.util.Base;
import com.tangosol.util.Binary;
import com.tangosol.util.ExternalizableHelper;


/**
 * DecorationOnlyDeltaCompressor is a DeltaCompressor which optimizes for
 * "decoration-only" updates.  A decoration-only update is a one where the
 * intrinsic value does not change, but the "decorations" may.  An example of
 * such an update could be generated by the ReadWriteBackingMap after successful
 * write-behind operations to indicate that the store has been completed.
 * Updates to the intrinsic value are not optimized/compressed at all.
 * 
 * While this compressor does not produce any compression/optimization in the
 * general case, it allows for optimizing decoration changes while avoiding the
 * need to "read" the old value when applying an actual intrinsic change.  This
 * could be used to defer/avoid unnecessary access to a potentially expensive
 * backup map.
 * 
 * DecorationOnlyBinaryDelta:
 *   DecorationDelta
 *   Value  // the new value in "whole" form (possibly decorated)
 *   null
 *
 * DecorationDelta:
 *   BIN_DECO_ONLY (decorated)    // apply the decorations to the old value
 *   BIN_DECO_ONLY (undecorated)  // undecorate the old value
 * 
 *
 * @author rhl 2011.08.09
 * @since  Coherence 3.7.1
 */
public class DecorationOnlyDeltaCompressor
        extends ExternalizableHelper
        implements DeltaCompressor
    {
    // ----- DeltaCompressor interface --------------------------------------

    /**
    * {@inheritDoc}
    */
    public ReadBuffer extractDelta(ReadBuffer bufOld, ReadBuffer bufNew)
        {
        if (bufOld == null)
            {
            // null old value means a "replace"
            return bufNew;
            }

        if (Base.equals(bufOld, bufNew))
            {
            // null means no change (see DeltaCompressor)
            return null;
            }

        boolean fDecoNew = isDecorated(bufNew);
        boolean fDecoOld = isDecorated(bufOld);
        if ((fDecoNew || fDecoOld) &&
            Base.equals(getUndecorated(bufNew), getUndecorated(bufOld)))
            {
            // we don't need to send the entire new value;
            // just the DECO_ONLY token decorated with the new set of
            // decorations (or undecorated for a "remove") will suffice
            Binary binDecoOnly = BIN_DECO_ONLY;
            if (fDecoNew)
                {
                ReadBuffer[] abufDeco = getDecorations(bufNew);

                // null out the 0th element to avoid overriding the value
                abufDeco[DECO_VALUE] = null;

                bufNew = decorate(binDecoOnly, abufDeco);
                }
            else
                {
                bufNew = binDecoOnly; // "remove the decorations" token
                }
            }

        return bufNew;
        }

    /**
    * {@inheritDoc}
    */
    public ReadBuffer applyDelta(ReadBuffer bufOld, ReadBuffer bufDelta)
        {
        if (bufDelta == null)
            {
            // null means no change (see DeltaCompressor)
            return bufOld;
            }

        if (BIN_DECO_ONLY.equals(getUndecorated(bufDelta)))
            {
            // decoration-only update; transfer or remove the decorations
            ReadBuffer[] abufDeco = getDecorations(bufDelta);

            // we only want the 'non-value' decorations
            abufDeco[DECO_VALUE] = null;

            bufOld = getUndecorated(bufOld);
            return abufDeco.length == 1 ? bufOld : decorate(bufOld, abufDeco);
            }
        else
            {
            // the "delta" is the new value in "whole" form
            return bufDelta;
            }
        }


    // ----- constants -------------------------------------------------------

    /**
     * A place-holder Binary value used by the compressor to indicate a
     * decoration-only update.
     */
    protected static Binary BIN_DECO_ONLY = new Binary(new byte[] {(byte) 0xFF});
    }